Page 406 - Rock Mechanics For Underground Mining

PILLAR SUPPORTED MINING METHODS

Figure 13.17 Relation between
yield of an orebody (given as
equivalent working height, h e ), room
span and real working height (after
Salamon, 1967).

the mining geometry may be assessed by considering an arbitrarily thick orebody,
selectingparticularstopespansandworkingheights,andcalculatingpillardimensions
(asdiscussedinoption(ii)intheprecedingdesignexercise)toachievearequiredfactor
of safety for the pillar-supported structure. Salamon carried out this type of exercise,
for the field conditions of a mining depth of 152 m, and a required factor of safety
of 1.6. The results of this exercise are presented in Figure 13.17, in which equivalent
working height is shown as a function of actual working height, for selected stope
spans. The observation from the plots is that independent increases in stope span
w o and real working height h both lead to increased equivalent working height, and
therefore to increased yield from the orebody. The significant engineering inference
is that recovery from an orebody may be maximised, while assuring the integrity of
the support system, if the following conditions are met simultaneously:

(a) the maximum (i.e. complete) thickness of the orebody is mined;
(b) the maximum room span consistent with assuring local stability of wall rock is
mined.

These conclusions may appear self-evident. The reality is that they are a direct result
of the nature of the pillar strength formula. A different relation between pillar strength
and pillar shape and dimensions could conceivably have led to different geometric
requirements for maximising volumetric yield from an orebody.
Having shown how maximum mineral potential of an orebody can be achieved in
a pillar-supported operation, it is useful to explore the way in which maximum yield
varies with geomechanical setting. The volumetric extraction ratio R can be seen,
from Figure 13.16b, to be given by the ratio of equivalent working height h e and
orebody thickness M; i.e. introducing equation 13.20

2
R = h e /M = h/M[1 − (w p /(w o + w p )) ] (13.21)
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